In a wireless communication system, it is often desirable to locate users or subscribers who are making calls. Applications for such location estimating technology may include locating subscribers that are requesting 911-emergency services, so that police, fire, or ambulance services may be efficiently dispatched to the user. Other applications for location technology include cellular fraud detection, aiding police investigations, billing based upon location, providing location-based information to the subscriber, and the like.
Known methods for estimating a subscriber's location in a cellular communications system include using a global positioning system (GPS) unit at the subscriber unit, using the time difference of signals arriving at multiple base stations to compute the subscriber's location, and recording and subsequently recognizing a subscriber's radio frequency signals from a plurality of known locations throughout the subscriber service area. While each of these known methods may work under certain circumstances, each of these methods have disadvantages.
For example, using a GPS receiver at the subscriber unit has the disadvantages of increasing the subscriber unit size, weight, and battery drain. Additionally, GPS units may be to costly to include in a competitively priced subscriber unit.
The disadvantage of using time difference of arriving signals is that it may not work in a cluttered area, such as a downtown or business district having tall buildings. In the cluttered area, signals are reflected and diffracted in a way that lengthens the propagation path between the subscriber unit and the base station. When the propagation path is lengthened, the time of arriving signals does not accurately represent the radial distance from the base station to the subscriber. Thus, conventional geometry and location algorithms will fail due to misleading times of arriving signals.
With regard to the system that recognizes previously recorded radio frequency signatures, disadvantages include the difficulty of taking enough measurements to produce an accurate database with a useful resolution, the susceptibility of the system to small changes in a receiving cell site configuration or small changes to the environment, and the difficulty of accurately determining the known locations that are recorded in the database.
Therefore, it should be apparent that a need exist for an improved method and system for estimating a subscriber's location in a wireless communications system service area, wherein expensive components are not required in the subscriber unit and wherein the subscriber's location may be accurately estimated in a cluttered environment without the need for manually measuring radio signals.